It is common practice to route communication cables and the like for computers, data devices, and alarm systems through plenums in building constructions. If a fire occurs in a building which includes plenums or risers, however, the non-fire retardant plenum construction would enable the fire to spread very rapidly throughout the entire building. Fire could travel along cables installed in the plenum, and smoke originating in the plenum could be conveyed to adjacent areas of the building.
A non-plenum rated cable sheath system, which encloses a core of insulated copper conductors, and which utilizes only a conventional plastic jacket, may not exhibit acceptable flame spread and smoke generation properties. As the temperature in such a cable rises due to a fire, charring of the jacket material may occur. If the jacket ruptures, the interior of the jacket and the insulation are exposed to elevated temperatures. Flammable gases can be generated, propagating flame and generating smoke.
Generally, the National Electrical Code requires that power-limited cables in plenums be enclosed in metal conduits. This is obviously a very expensive construction due to the cost of materials and labor involved in running conduit or the like through plenums. The National Electrical Code does, however, permit certain exceptions to the requirements so long as such cables for plenum use are tested and approved by an independent testing laboratory, such as the Underwriters Laboratory (UL), as having suitably low flame spread and smoke-producing characteristics. The flame spread and smoke production characteristics of plenum cable are tested and measured per the UL-910 plenum burn standard.
With plenum cables, in addition to concerns about flammability and smoke production, the cables must also, of course, have suitable electrical characteristics for the signals intended to be carried by the cables. There are various categories of cable, such as Category 3, Category 4, Category 5, etc., with increasing numbers referring to enhanced or higher frequency electrical transmission capabilities. With Category 5, for example, extremely good electrical parameters are required, including low attenuation, structural return loss, and cross-talk values for frequencies up to 100 MHz. Unfortunately, cable materials which generally have the requisite resistance to flammability and smoke production also result in electrical parameters for the cable generally not suitable for the higher transmission rates, such as a Category 5 cable. Specifically, Category 5 plenum cables must: (1) pass the UL-910 plenum burn test; (2) pass physical property testing set forth in the UL-444 standard relating to communications cables; and (3) meet the Category 5 electrical requirements such as provided in Electronic Industries Association specification TIA/EIA-568A.
Currently, a cable construction which is available and which meets these requirements is provided in a configuration which includes fluorinated ethylene propylene (FEP) as insulation, with a low-smoke polyvinyl chloride (PVC) jacket. Such a cable construction meets the 100 MHz frequency operation requirements, and it has been demonstrated that such a cable construction can be suitable for asynchronous transfer mode (ATM) applications. Unfortunately, FEP at times may be in short supply. Given the manufacturing capacity of FEP producers, only enough FEP is currently produced to meet approximately 80 percent of the demand for the volume of material required to construct high-category cables. Although it could be expected that the supply of FEP will continue to increase, it is apparent that the available quantity of FEP may not meet the demand for the material for use in plenum cables as the domestic market is projected to increase at a rate of approximately 20 percent per year in the near future, and the potential use of such Category 5 plenum cables in European and Scandinavian markets may further increase the demand for FEP.
One current riser cable utilizes a foam/skin insulation. The insulation material construction is a foamed, high density polyethylene and PVC skin composite. A jacketed and shielded cable of this insulation core can be designed to meet the Category 3 electrical and the plenum burn requirements. However, developing a Category 5 plenum cable is very difficult due to the extreme electrical parameters necessary, e.g., attenuation, structural return loss, and cross-talk values to 100 MHz. Furthermore, this core must pass elevated temperature attenuation requirements at 40.degree. C. and 60.degree. C. The above-mentioned insulation composite with a PVC skin will not pass the elevated temperature attenuation requirements because the dielectric constant of PVC increases with temperature.